I'm going to take a non-end-experience approach to commeting on this.
I'm not enthusiastic about Silverlight as it doesn't have, for example, real windowing. I would say that using Silverlight as the interface is a great idea if it means greater development productivity for the MS Dev and MS QA teams to produce the same (or more/better) UI. If DHTML/AJAX is more productive and quality-testable for the same basic tasks, stick with it.
I come from a mindset where I believe that a product is only as good as the developers' experience was, because the end product its their fruit. For example, a *good* DHTML/AJAX developer not hampered by the oil+water scenario of ASP.NET Web Forms + custom Javascript can get much done very fast and produce a very high quality product. Silverlight's developers are new to productive development workflow because Silverlight itself is still relatively new.